Solid state deformation processing of crosslinked high molecular weight polymeric materials

ABSTRACT

Solid-state deformation processing of crosslinked high molecular weight polymers such as UHMWPE, for example by extrusion below the melt transition, produces materials with a combination of high tensile strength and high oxidative stability. The materials are especially suitable for use as bearing components in artificial hip and other implants. Treated bulk materials are anisotropic, with enhanced strength oriented along the axial direction. The material is oxidatively stable even after four weeks of accelerated aging in a pressure vessel containing five atmospheres of oxygen (ASTM F2003). Because of its oxidative stability, the deformation processed material is a suitable candidate for air-permeable packaging and gas sterilization, which has thus far been reserved for remelted crosslinked UHMWPE.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/963,974 filed Oct. 13, 2004, now U.S. Pat. No. 7,344,672, whichclaims the benefit of U.S. Provisional Application Serial No. 60/616,811filed Oct. 7, 2004, the entire disclosure of which is herebyincorporated by reference.

INTRODUCTION

The invention relates to crosslinked high molecular weight polymericmaterial and methods for treating the materials to provide enhancedproperties. In particular, the invention provides methods and materialsfor use in preparing polymeric implants with a high degree of wear andoxidation resistance.

Crosslinked ultra high molecular weight polyethylene (UHMWPE) is nowwidely used in medical implants such as acetabular components for totalhip replacements. There remains interest by the orthopedic community tofind alternative methods of processing radiation crosslinked UHMWPE toimprove mechanical properties while still retaining wear resistance andoxidative stability in the material.

In U.S. Pat. No. 6,168,626, Hyon et al. report enhancement of themechanical properties of crosslinked UHMWPE by deformation processing ata compression deformable temperature. After deformation processing, thematerial is cooled while keeping the deformed state. An oriented UHMWPEmolded article is obtained that has an orientation of crystal planes ina direction parallel to the compression plane. The compression iscarried out using a suitable die or can be done using a hot pressmachine.

Polymeric materials such as UHMWPE can be crosslinked to providematerials with superior wear properties, for example. The polymericmaterials may be chemically crosslinked or preferably crosslinked withirradiation such as γ-irradiation. The action of γ-irradiation on thepolymer results in the formation of free radicals within the bulkmaterials. The free radicals provide sites for reactions to crosslinkthe molecular chains of the bulk materials. It has become recognizedthat the presence of free radicals, including any free radicals thatsurvive after subsequent heat treatment, are also susceptible to attackby oxygen to form oxidation products. The formation of such oxidationproducts generally leads to deterioration of mechanical properties.

To completely remove free radicals and provide polymeric materials ofhigh oxidative stability, it is common to heat treat the crosslinkedmaterial above the crystalline melting point of the polymer. This has atendency to destroy or recombine all of the free radicals in the bulkmaterial. As a result, the crosslinked material is highly resistant tooxidative degradation. However, some desirable mechanical properties arelost during the melting step.

It would be desirable to provide materials such as crosslinked UHMWPEthat combine a high level of mechanical properties and a high resistanceto oxidative degradation.

SUMMARY

A method of solid state deformation processing of crosslinked polymersincludes deforming a polymer bulk material by compressing it in adirection orthogonal to a main axis of the bulk material and optionallycooling the bulk material while maintaining the deformation force. Whenthe polymeric material is made of UHMWPE and the crosslinking is byirradiation such as γ-irradiation, products of the method areparticularly suitable for use in bearing components and implants fortotal hip replacement and the like.

In one aspect, the invention involves solid state extrusion of anelongate bulk material through a reducing die while the material is at acompression deformable temperature, preferably below the melting point.The extruded bulk material is then cooled, preferably while held in thedeformed state. After cooling, the bulk material is stress relieved byreheating to an annealing temperature to below the melting point, thistime without applying pressure.

An oriented UHMWPE molded article can be obtained according to methodsof the invention by crosslinking a UHMWPE raw article with a high energyray such as gamma-irradiation, heating the crosslinked UHMWPE to acompression deformable temperature, and compression deforming theUHMWPE, followed by cooling and solidifying. The material has adetectable level of free radicals and yet is resistant to oxidativedegradation evidenced by a very low, preferably undetectable, increasein infrared absorption bands of the UHMWPE material that correspond toformation of carbonyl groups during accelerated aging.

By compression deforming in a direction orthogonal to the main axis of abulk material, an anisotropic material is formed wherein mechanicalproperties in the direction of the main axis differ from mechanicalproperties in the orthogonal or transverse direction. After stressrelieving, mechanical properties can differ by 20% or more in the axialdirection as opposed to the orthogonal directions.

Polymers treated by the methods exhibit a desirable combination of hightensile strength and resistance to oxidative degradation. Transversedeformation of UHMWPE, for example, leads to material having a tensilestrength at break greater than 50 Mpa and preferably greater than 60Mpa, measured in the axis orthogonal to the deformation. At the sametime, the material is resistant to oxidative degradation, showing inpreferred embodiments essentially no change in oxidation index onaccelerated aging.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 illustrates the geometry of an extrusion process;

FIG. 2 shows various embodiments of extrusion apparatus and dies; and

FIG. 3 illustrates an embodiment of an extrusion process.

DESCRIPTION

The headings (such as “Introduction” and “Summary,”) used herein areintended only for general organization of topics within the disclosureof the invention, and are not intended to limit the disclosure of theinvention or any aspect thereof. In particular, subject matter disclosedin the “Introduction” may include aspects of technology within the scopeof the invention, and may not constitute a recitation of prior art.Subject matter disclosed in the “Summary” is not an exhaustive orcomplete disclosure of the entire scope of the invention or anyembodiments thereof. Similarly, subpart headings in the Description aregiven for convenience of the reader, and are not a representation thatinformation on the topic is to be found exclusively at the heading.

The description and specific examples, while indicating embodiments ofthe invention, are intended for purposes of illustration only and arenot intended to limit the scope of the invention. Moreover, recitationof multiple embodiments having stated features is not intended toexclude other embodiments having additional features, or otherembodiments incorporating different combinations of the stated features.Specific Examples are provided for illustrative purposes of how to make,use and practice the compositions and methods of this invention and,unless explicitly stated otherwise, are not intended to be arepresentation that given embodiments of this invention have, or havenot, been made or tested.

As used herein, the words “preferred” and “preferably” refer toembodiments of the invention that afford certain benefits, under certaincircumstances. However, other embodiments may also be preferred, underthe same or other circumstances. Furthermore, the recitation of one ormore preferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the invention.

As used herein, the word “include,” and its variants, is intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that may also be useful in the materials,compositions, devices, and methods of this invention.

In one embodiment, the invention provides a method for treating apolymeric bulk material comprising heating a crosslinked polymer to acompression deformable temperature, applying force to deform the heatedpolymer, and cooling the polymer to a solidification temperature whilemaintaining the polymer in a deformed state. The crosslinked polymer isin a bulk form characterized by an axial direction; force is applied todeform the heated polymer in a direction orthogonal to the axialdirection. Among other desirable physical properties, the polymeric bulkmaterial as treated by the above method exhibits enhanced strength inthe axial direction of the bulk material. When the bulk material isUHMWPE, the method is especially suitable for providing a medicalimplant containing bearing components made out of the UHMWPE.

In another embodiment, the invention provides a method for treatingcrosslinked UHMWPE for making material suitable for use in a medicalimplant. The method involves heating UHMWPE to a temperature above about80° C. and below its melting point, where the UHMWPE has beencrosslinked with γ-irradiation. The UHMWPE is in the form of a bulkmaterial characterized by an axial direction, a transverse directionorthogonal to the axial direction, and an original dimension.Compressive force is then applied on the bulk material in the transversedirection to the reduce a dimension of the bulk material in thatdirection. Then the bulk UHMWPE is cooled to a solidificationtemperature. In one embodiment, force is applied during the coolingsufficient to prevent the bulk material from returning to its originaldimension. In various embodiments, compressive force is applied by ramextruding the bulk material through a reducing die, for instance througha circular die, with a diametral compression or draw ratio greater than1.

In various embodiments, compressive force is maintained on the coolingUHMWPE by extruding the heated crosslinked bulk material into a chamberof sufficient size and shape to hold the bulk material at a dimension inthe transverse direction less than original radial dimension

In particular embodiments, the invention provides a method for preparinga preform made of UHMWPE suitable for use in medical implants. Themethod comprises heating a γ-irradiation crosslinked UHMWPE rodcharacterized by a crystalline melting point and a diameter d₁ to acompression deformable temperature. Thereafter, compression force isapplied on the crosslinked UHMWPE to decrease the diameter to d₂,wherein d₂ is less than d₁. The reduced diameter rod of UHMWPE isoptionally cooled to a solidification temperature while maintainingcompression force to keep the diameter at a value of d₃, wherein d₃ isless than d₁. In a subsequent step, the cooled rod is stress relieved byheating to a temperature at which the rod expands to a diameter d₄,wherein d₄ is greater than d₃. In various embodiments, the methodinvolves extruding the rod through a reducing die into a cooling chamberto reduce the diameter from d₁ to d₂. The compression deformabletemperature is preferably less than the melting point and greater than atemperature equal to the melting point minus 50° C. In a preferredembodiment, the compression deformable temperature is from about 100° C.to about 135° C. Preferably, the UHMWPE rod has been crosslinked withγ-irradiation at a dose from 0.1 to 10 Mrad. Methods are also providedfor making a bearing component for a medical implant from UHMWPE treatedaccording to the method above, as well as implants comprising a UHMWPEmachined from a preform made according to the above methods.

In one aspect, the invention provides a γ-crosslinked UHMWPE in the formof an elongate material, such as a cylinder, characterized by an axialdirection. The tensile strength in the axial or longitudinal directionis greater than 50 MPa and preferably greater than 60 MPa. In preferredembodiments, bearing components comprise UHMWPE machined or formed fromsuch a γ-crosslinked UHMWPE. Medical implants contain the bearingcomponents.

In another aspect, the invention provides a γ-crosslinked UHMWPE havinga detectable concentration level of free radicals, but neverthelessstable to oxidation as measured by standard tests. For example, in anon-limiting example, the concentration of free radicals in the UHMWPEis above about 0.06×10¹⁵ spins/g and below about 3×10¹⁵ spins/g.Preferably, the free radical concentration is 1.5×10¹⁵ spins/g or less.In a preferred embodiment, there is no detectable increase in thecarbonyl IR absorption band during exposure to oxygen at 5 atmospheresfor four weeks at 70° C. The crosslinked UHMWPE is advantageouslyprovided in the form of a cylindrical rod having a diameter of about 2to 4 inches and preferably about 3 inches. Bearing components areprovided by machining the components from the crosslinked UHMWPE, andmedical implants are provided that contain the bearing components.

In a further aspect, an anisotropic crosslinked UHMWPE is provided inthe form of a bulk material characterized by an axial direction and atransverse direction orthogonal to the axial direction. In variousembodiments, the anisotropy is characterized in that the tensilestrength in the axial direction is 20% or more greater than the tensilestrength in the radial direction, and attains the value of at least 50MPa, preferably at least 60 MPa.

In a further aspect, a method for solid state deformation processing ofγ-irradiated crosslinked UHMWPE comprises deforming the UHMWPE byextruding it at a temperature below its melt transition or crystallinemelting point. In subsequent steps, the extruded UHMWPE is cooled to atemperature below its solidification temperature, optionally whilemaintaining the extruded rod in a deformed state.

In other embodiments, a compression deformed crosslinked UHMWPE having atensile strength at break of more than 50 MPa is provided by treatingUHMWPE according to the methods. In preferred embodiments, the materialis also resistant to oxidative degradation, characterized by anoxidation index less than 0.5 after exposure to 5 atm of oxygen at 70°C. for 4 days, in spite of the material having a detectable free radicalconcentration above 0.06×10¹⁵ spins/g.

In another aspect, the invention provides a method for making a medicalimplant containing a bearing component made of UHMWPE. The methodincludes the steps of radiation crosslinking a UHMWPE in the form of abulk material, preheating the crosslinked UHMWPE to a temperature above80° C. and below its melting point, then solid state extruding thepreheated UHMWPE to a diametral compression ratio of greater than 1,cooling the extruded UHMWPE to a solidification temperature below 30° C.while maintaining diametral compression, annealing the cooled UHMWPE ata temperature below the melting point for a time sufficient for the rodto increase in diameter in response to the annealing, and machining thebearing component from the annealed UHMWPE. The UHMWPE is optionallysterilized after machining the bearing component. Sterilizing ispreferably performed by non-irradiative means such as exposure to gasessuch as ethylene oxide.

In various embodiments, implants are manufactured using preformedpolymeric compositions having the structures described herein and madeby the methods described herein. Non-limiting examples of implantsinclude hip joints, knee joints, ankle joints, elbow joints, shoulderjoints, spine, temporo-mandibular joints, and finger joints. In hipjoints, for example, the preformed polymeric composition can be used tomake the acetabular cup or the insert or liner of the cup. In the kneejoints, the compositions can be made used to make the tibial plateau,the patellar button, and trunnion or other bearing components dependingon the design of the joints. In the ankle joint, the compositions can beused to make the talar surface and other bearing components. In theelbow joint, the compositions can be used to make the radio-numeral orulno-humeral joint and other bearing components. In the shoulder joint,the compositions can be used to make the glenero-humeral articulationand other bearing components. In the spine, intervertebral discreplacements and facet joint replacements may be made from thecompositions.

In various embodiments, the bearing components are made from thepolymeric compositions by known methods such as by machining and areincorporated into implants by conventional means.

Polymers

For implants, preferred polymers include those that are wear resistant,have chemical resistance, resist oxidation, and are compatible withphysiological structures. In various embodiments, the polymers arepolyesters, polymethylmethacrylate, nylons or polyamides,polycarbonates, and polyhydrocarbons such as polyethylene andpolypropylene. High molecular weight and ultra high molecular weightpolymers are preferred in various embodiments. Non-limiting examplesinclude high molecular weight polyethylene, ultra high molecular weightpolyethylene (UHMWPE), and ultra high molecular weight polypropylene. Invarious embodiments, the polymers have molecular ranges from approximatemolecular weight range in the range from about 400,000 to about10,000,000.

UHMWPE is used in joint replacements because it possesses a lowco-efficient of friction, high wear resistance, and compatibility withbody tissue. UHMWPE is available commercially as bar stock or blocksthat have been compression molded or ram extruded. Commercial examplesinclude the GUR series from Hoechst. A number of grades are commerciallyavailable having molecular weights in the preferred range describedabove.

Crosslinking

According to various embodiments of the invention, a crosslinkedpolymeric bulk material is further processed in a series of heating,deforming, cooling, and machining steps. The polymeric bulk material canbe crosslinked by a variety of chemical and radiation methods.

In various embodiments, chemical crosslinking is accomplished bycombining a polymeric material with a crosslinking chemical andsubjecting the mixture to temperature sufficient to cause crosslinkingto occur. In various embodiments, the chemical crosslinking isaccomplished by molding a polymeric material containing the crosslinkingchemical. The molding temperature is the temperature at which thepolymer is molded. In various embodiments, the molding temperature is ator above the melting temperature of the polymer.

If the crosslinking chemical has a long half-life at the moldingtemperature, it will decompose slowly, and the resulting free radicalscan diffuse in the polymer to form a homogeneous crosslinked network atthe molding temperature. Thus, the molding temperature is alsopreferably high enough to allow the flow of the polymer to occur todistribute or diffuse the crosslinking chemical and the resulting freeradicals to form the homogeneous network. For UHMWPE, a preferredmolding temperature is between about 130° C. and 220° C. with a moldingtime of about 1 to 3 hours. In a non-limiting embodiment, the moldingtemperature and time are 170° C. and 2 hours, respectively.

The crosslinking chemical may be any chemical that decomposes at themolding temperature to form highly reactive intermediates, such as freeradicals, that react with the polymers to form a crosslinked network.Examples of free radical generating chemicals include peroxides,peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, anddivinylbenzene. Examples of azo compounds are: azobis-isobutyronitrile,azobis-isobutyronitrile, and dimethylazodi-isobutyrate. Examples ofperesters are t-butyl peracetate and t-butyl perbenzoate.

Preferably the polymer is crosslinked by treating it with an organicperoxide. Suitable peroxides include2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne (Lupersol 130, AtochemInc., Philadelphia, Pa.); 2,5-dimethyl-2,5-di-(t-butylperoxy)-hexane;t-butyl α-cumyl peroxide; di-butyl peroxide; t-butyl hydroperoxide;benzoyl peroxide; dichlorobenzoyl peroxide; dicumyl peroxide;di-tertiary butyl peroxide; 2,5-dimethyl-2,5-di(peroxybenzoate)hexyne-3; 1,3-bis(t-butyl peroxy isopropyl)benzene; lauroylperoxide; di-t-amyl peroxide; 1,1-di-(t-butylperoxy)cyclohexane;2,2-di-(t-butylperoxy)butane; and 2,2-di-(t-amylperoxy)propane. Apreferred peroxide is 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne.The preferred peroxides have a half-life of between 2 minutes to 1 hour;and more preferably, the half-life is between 5 minutes to 50 minutes atthe molding temperature.

Generally, between 0.2 to 5.0 wt % of peroxide is used; more preferably,the range is between 0.5 to 3.0 wt % of peroxide; and most preferably,the range is between 0.6 to 2 wt %.

The peroxide can be dissolved in an inert solvent before being added tothe polymer powder. The inert solvent preferably evaporates before thepolymer is molded. Examples of such inert solvents are alcohol andacetone.

For convenience, the reaction between the polymer and the crosslinkingchemical, such as peroxide, can generally be carried out at moldingpressures. Generally, the reactants are incubated at moldingtemperature, between 1 to 3 hours, and more preferably, for about 2hours.

The reaction mixture is preferably slowly heated to achieve the moldingtemperature. After the incubation period, the crosslinked polymer ispreferably slowly cooled down to room temperature. For example, thepolymer may be left at room temperature and allowed to cool on its own.Slow cooling allows the formation of a stable crystalline structure.

The reaction parameters for crosslinking polymers with peroxide, and thechoices of peroxides, can be determined by one skilled in the art. Forexample, a wide variety of peroxides are available for reaction withpolyolefins, and investigations of their relative efficiencies have beenreported. Differences in decomposition rates are perhaps the main factorin selecting a particular peroxide for an intended application.

Peroxide crosslinking of UHMWPE has also been reported. UHMWPE can becrosslinked in the melt at 180° C. by means of2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexyne-3.

In various embodiments, crosslinking is accomplished by exposing apolymeric bulk material to irradiation. Non-limiting examples ofirradiation for crosslinking the polymers include electron beam, x-ray,and gamma-irradiation. In various embodiments, gamma irradiation ispreferred because the radiation readily penetrates the bulk material.Electron beams can also be used to irradiate the bulk material. Withe-beam radiation, the penetration depth depends on the energy of theelectron beam, as is well known in the art.

For gamma (γ) irradiation, the polymeric bulk material is irradiated ina solid state at a dose of about 0.01 to 100 Mrad (0.1 to 1000 kGy),preferably from 0.01 to 10 MRad, using methods known in the art, such asexposure to gamma emissions from an isotope such as ⁶⁰Co. In variousembodiments, gamma irradiation is carried out at a dose of 0.01 to 6,preferably about 1.5 to 6 Mrad. In a non-limiting embodiment,irradiation is to a dose of approximately 5 MRad.

Irradiation of the polymeric bulk material is usually accomplished in aninert atmosphere or vacuum. For example, the polymeric bulk material maybe packaged in an oxygen impermeable package during the irradiationstep. Inert gases, such as nitrogen, argon, and helium may also be used.When vacuum is used, the packaged material may be subjected to one ormore cycles of flushing with an inert gas and applying the vacuum toeliminate oxygen from the package. Examples of package materials includemetal foil pouches such as aluminum or Mylar® coating packaging foil,which are available commercially for heat sealed vacuum packaging.Irradiating the polymeric bulk material in an inert atmosphere reducesthe effect of oxidation and the accompanying chain scission reactionsthat can occur during irradiation. Oxidation caused by oxygen present inthe atmosphere present in the irradiation is generally limited to thesurface of the polymeric material. In general, low levels of surfaceoxidation can be tolerated, as the oxidized surface can be removedduring subsequent machining.

Irradiation such as γ-irradiation can be carried out on polymericmaterial at specialized installations possessing suitable irradiationequipment. When the irradiation is carried out at a location other thanthe one in which the further heating, compressing, cooling, andmachining operations are to be carried out, the irradiated bulk materialis conveniently left in the oxygen impermeable packaging during shipmentto the site for further operations.

Bulk Form of the Materials

The crosslinked polymer is provided in a bulk form characterized by anaxial direction and a transverse direction orthogonal or perpendicularto the axial direction. In subsequent processing steps, deformationpressure is applied on the crosslinked bulk material to reduce adimension in the transverse direction.

The axial direction is also the direction in which high tensile strengthis developed, as described further below. In this aspect, the axialdirection of the bulk material is the direction perpendicular to theapplication of the deformation force that leads to development of hightensile strength in the axial direction. In this way, application ofdeformation pressure or force orthogonal to the axial direction createsan anisotropic material, characterized by higher tensile strength in theaxial than in the transverse direction.

The axial direction of the bulk material also defines the preferreddirection in which implant bearing components such as acetabular cupsare to be machined. That is, bearing components are preferably made ormachined from the treated bulk polymer in an orientation where the hightensile strength axis of the polymer corresponds to the load bearingaxis or direction of the bearing component of the implant in vivo.

In an exemplary embodiment, the bulk material is in the form of a rod orcylinder having a circular cross section. The axial direction isparallel to the main axis of the cylinder, while the transversedirection is at right angles to the axial direction. In other words, theexistence of the axial direction defines an orthogonal directionreferred to as “transverse” in this application. When the cross sectionof the bulk material is isotropic as in the case of a cylinder, thetransverse direction can be described as “radial”, and the transverseaxis as a radial axis. The main axis of the bulk material can also becalled the longitudinal axis. As used here, the longitudinal axis isparallel to the axial direction.

In the non-limiting case of a rod or cylinder, a cross section of thebulk material perpendicular to the axial direction or longitudinal axisis a circle. Other bulk materials characterized by an axial directionmay be used that have other perpendicular cross sections. In anon-limiting example, a square cylinder can be provided that has asquare cross section perpendicular to the axial direction. Other bulkmaterials characterized by an axial direction can have rectangular,polygonal, star, lobed, and other cross sections perpendicular to theaxial direction.

In various embodiments, the axial direction of the bulk polymericmaterial is elongated compared to the orthogonal or radial direction.For example, in the case of UHMWPE, a commercially available bulkmaterial is a cylinder approximately 3 inches in diameter and 14 inchesin length. The length corresponds to the axial direction and thediameter corresponds to the radial direction. As described below,bearing components for implants are preferably machined from billets cutin the axial direction. For efficiency in manufacturing it is convenientto produce a number of bearing components from a single bulk materialtreated by the methods of the invention. For this reason, the bulkmaterial is usually to be extended in an axial direction so as to beable to cut a plurality of billets from the material for use in furthermachining of the bearing components.

As described above, bulk material characterized by an axial direction isfurther characterized as having a variety of cross sectional areasperpendicular to the axial direction. In various embodiments, thedimensions of the cross sectional areas perpendicular to the axialdirection are more or less constant along the axial direction from thebeginning to the end or from the top to the bottom of the bulk material.In various other embodiments, bulk materials may be provided to havecross sectional areas that vary along the length or axial direction ofthe bulk material. In the case where the cross sectional area of thebulk material is constant along the axial direction of the bulkmaterial, compressive force applied as described below will generally beapplied to the bulk material in a direction perpendicular to the axialdirection. In the case where the cross sectional area varies along theaxial direction of the bulk material, compressive force applied to thebulk material may have a component in the axial direction due to thegeometry of the bulk material. However, in all cases at least acomponent of the compressive force will be applied on the bulk materialin a direction orthogonal to the axial direction.

Pre-Heating

Before further processing, the crosslinked polymer is heated to acompression deformable temperature. The compression deformabletemperature is temperature at which the polymeric bulk material softensand can flow under the application of a compressive source to changedimension in the direction the compressive force is applied. For UHMWPEand other polymeric materials, the compression deformable temperature isconcretely from about the melting point minus 50° C. to the meltingpoint plus 80° C.

In various embodiments, the compression deformable temperature is belowthe melting point of the polymeric material. Examples of the compressiondeformable temperature include from the melting point to 10° C. belowthe melting point, from the melting point to 20° C. below the meltingpoint, from the melting point to 30° C. below the melting point, andfrom the melting point to 40° C. below the melting point. For UHMWPE,the compression deformable temperature is above 80° C., or from about86° C. to about 136° C., since the melting temperature of the UHMWPE isabout 136° C. to 139° C. In various embodiments, the compressiondeformable temperature of UHMWPE lies from about 90° C. to 135° C.,preferably about 100° C. to 130° C. A preferred temperature is 125-135°C., or 130° C.±5° C.

In various embodiments, the crosslinked material is heated to acompression deformable temperature above the melting point of thepolymer. For UHMWPE and other polymeric materials, such a compressiondeformable temperature is from just above the melting point to atemperature about 80° C. higher than the melting point. For example,UHMWPE can be heated to a temperature of 160° C. to 220° C. or 180° C.to 200° C.

In various embodiments, it is preferred to heat the bulk polymericmaterial to a compression deformation temperature close to but nothigher than the melting point. In various embodiments, the compressiondeformable temperature is between the melting point and a temperature20° C. lower than the melting point, or between the melting point and atemperature 10° C. lower than the melting point.

The crosslinked bulk material can be heated to a compression deformabletemperature in a deformation chamber as illustrated in the figures, orit can be preheated in an oven to the compression deformabletemperature. In various embodiments, the bulk material is heated to atemperature just below the melting point, such as the melting pointminus 5° or the melting point minus 10° and placed in a heateddeformation chamber. The deformation chamber preferably maintains acompression deformable temperature. If desired, the deformation chambercan be heated or thermostatted to maintain a constant temperature.Alternatively, the deformation chamber is not itself heated but hassufficient insulating properties to maintain the bulk material at acompression deformable temperature during the course of extrusionthrough the reducing die described below. In various embodiments, thetemperature of the deformation chamber is held at several degrees belowthe melting temperature to avoid melting.

Deformation

When the crosslinked bulk material is at a compression deformabletemperature, deforming pressure is applied to the bulk material in adirection orthogonal to the axial direction. The application of theorthogonal force results in material flow of the heated bulk material.As a result, a dimension of the bulk material in the transversedirection at which force is applied is diminished compared to theoriginal dimension. As discussed above, compression force is applied sothat least one component of the force is orthogonal to the axialdirection of the bulk material. For cylindrical rods and other bulkmaterials that have a constant cross section along the axial directionof the bulk material, the compression force is applied in a directionperpendicular to the axial direction.

Any suitable methods may be used to apply the compression force in adirection orthogonal to the axial direction. Non-limiting examplesinclude rollers, clamps, and equivalent means.

Extrusion

In various embodiments, deforming force is applied in the directionalorthogonal to the axial direction of the bulk material by extruding thebulk material through a reducing die. Pressure exerted on the bulkmaterial in a direction orthogonal to the axial direction duringextrusion causes the dimension of the bulk material to be reducedcompared to the original dimension of the bulk material. In other words,the diameter or other transverse dimension of the bulk material afterextrusion is less than the dimension before extrusion.

The relative reduction in the dimension of the bulk material in thetransverse directions can be expressed as a ratio of the originaldimension d₁ to the reduced dimension d₂. Depending on the method ofreducing the dimension by applying compressive force, the numeric valueof the ratio d₁/d₂ can be referred to as a draw ratio or a diametralcompression. For extrusion, it is common practice to refer to a drawratio; unless stated otherwise from context, the term draw ratio will beused here to refer to all geometries.

It is to be understood that the transverse direction (the directionorthogonal to the axial direction) in which deformation pressure orforce is applied itself contains two axes that can be drawn at rightangles to the longitudinal axis. In various embodiments, the bulkmaterial can be deformed by a different amount along the two transverseaxes, and a draw ratio can be defined for both axes. The orientation ofthe transverse axes is arbitrary; if needed for analysis, the axes canbe selected to simplify the geometry of the applied forces. When thecross section of the bulk material is isotropic, equal deformation forcecan be applied in all transverse directions. In this non-limiting case,the dimension d₂ corresponds to the radius or diameter of the extrudedmaterial, and the draw ratio is the fraction defined by dividing d₁ byd₂.

In various embodiments, the draw ratio is 1.1 or higher, and less thanabout 3. In various embodiments, the draw ratio is 1.2 or higher, and ispreferably about 1.2 to 1.8. It is about 1.5 in a non-limiting example.At high levels of reduction, a point is reached at which the strainintroduced is too great and the properties of the crosslinked polymericmaterials deteriorate. Accordingly, in various embodiments the drawratio is 2.5 or less, and preferably about 2.0 or less. In a preferredembodiment, the compressive force is applied more or less isotropicallyaround the bulk material in a direction transverse to a longitudinalaxis. Accordingly, the reduction in dimension will usually apply in alltransverse directions. To illustrate, a circular cross section remainsround but is reduced in diameter, while a polygonal cross section suchas a square or rectangle is reduced on all sides.

The geometry of extrusion through a reducing die is illustrated inschematic form in FIGS. 1 and 2. A reducing die 6 is disposed between adeformation chamber 2 and a cooling chamber 4. As shown, the reducing 6die serves to reduce the diameter or dimension of the extruded rod froman original dimension d₁ to an extruded dimension d₂. As the crosslinkedheated bulk material passes from the deformation chamber through thereducing die 6, the material flows by the die wall 5 that leads to aconstriction 10 having the diameter d₂ of the cooling chamber 4.

Various geometries of the reducing die are illustrated in non-limitingform in FIG. 2. FIGS. 2 a to 2 e show the relative configuration of thedeformation chamber wall 20 and the cooling chamber wall 10. The diewall 5 is seen to connect the cooling deformation chamber to the coolingchamber. In FIG. 2 a, the cross section of both the deformation chamber2 and cooling chamber 4 are circular, with dimensions d₁ and d₂corresponding to their respective diameters. In FIG. 2 b, thedeformation chamber is square or rectangular characterized by adimension d1 that can be arbitrarily taken along a diagonal or along aside. In FIG. 2 b, the cooling chamber 4 is also rectangular but havinglower dimension d₂. FIGS. 2 c through 2 e illustrate other combinationsof circular, square, and triangular deformations and cooling chambersconnected by reducing dies 6 having a die wall 5, and are offered by wayof non-limiting example.

As noted above, the bulk material in the deformation chamber 2 is heldat a compression deformable temperature. At such a temperature, thematerial can flow in response to pressure exerted on the material. Whenthe compression deformable temperature is below the melting point, thematerial undergoes a solid state flow through the reducing die 6.Pressure or force applied to the end of the bar by the ram is translatedby the die into compressive force that reduces the dimension of the bulkmaterial in the transverse direction. Conveniently, the diameter of thebulk material to be extruded matches relatively closely the diameter ordimension d₁ of the deformation chamber illustrated in FIG. 1.

Cooling

In various embodiments, an extruded UHMWPE rod or other crosslinkedpolymeric material in a bulk form characterized by an axial direction iscooled before further processing. Alternatively, the extruded bulkmaterial can be directly processed by the stress relief step describedbelow. In a non-limiting embodiment, the rod or other bulk materialcharacterized by an axial direction is cooled to a solidificationtemperature in a cooling chamber or other means while pressure ismaintained sufficient to keep the dimension of the extruded bulkmaterial below the original dimension of the crosslinked bulk material.In the extrusion or other compressive force embodiments, the pressurerequired to maintain the dimension lower than the original dimension maybe more or less pressure than required to originally change the shape ofthe polymer, such as through extrusion. As noted, the bulk material suchas extruded UHMWPE is held in a cooling chamber or similar device for asufficient time to reach a temperature at which the bulk material nolonger has a tendency to increase in dimension upon removal of thepressure. This temperature is designated as the solidificationtemperature; for UHMWPE the solidification temperature is reached when athermostat embedded in the cooling wall (about 1 mm from the inside wallsurface) reads about 30° C. The solidification temperature is not aphase change temperature such as a melting or freezing. It is also to benoted that a material such as UHMWPE can be cooled to the solidificationtemperature independently of whether the material was heated above orbelow the melting point in a previous processing step.

In various embodiments, after extrusion or other application ofdeforming force in a direction orthogonal to the axial direction of thecrosslinked polymeric bulk material, the compressive deforming force ismaintained on the bulk material until the bulk material cools to thesolidification temperature. Such a maintenance of compressive force isconveniently provided in the reducing die embodiment illustrated inFIGS. 1 and 2. After extrusion through the reducing die 6, the bulkmaterial is held in the cooling chamber 4. In the embodiment shown inthe Figures, the cooling chamber is of such a size and shape as to holdthe extruded bulk material at a dimension or diameter d₃, which is lessthan the original dimension d₁ of the bulk material and is convenientlyabout the same as the extruded dimension d₂ in a non-limiting example.The crosslinked material has a tendency to return to its originaldimension by expanding when the temperature is above the solidificationtemperature. The expansion force of the bulk material is counteracted bythe walls of the cooling chamber, with the result that compressive forceis maintained on the bulk material while it cools. In variousembodiments, the cooling chamber is provided with cooling means such ascooling jackets or coils to remove heat from the cooling chamber and theextruded polymer bulk material.

Referring to the figures for illustration, as the polymeric extrudedbulk material cools in the cooling chamber, a temperature is reached atwhich the material no longer has a tendency to expand or revert to itsoriginal dimension d₁. At this temperature, called the solidificationtemperature, the bulk material no longer exerts pressure on the walls ofthe cooling chamber and can be removed. In preferred embodiments, thematerial is cooled to about 30° C., as measured by thermostats in thewalls of the chamber, before removal.

The temperatures of the deformation chamber and the cooling chamber canbe measured by conventional means, such as by thermocouples embeddedinto the walls of the respective chambers. For example, it has beenfound that when a thermocouple in the wall of the cooling chamberindicates a temperature of 30° C., an extruded bulk material made ofUHMWPE has reached a bulk temperature below a solidification temperatureat which the material loses it tendency to expand. The temperature asmeasured with, for example, a thermocouple embedded in the wall of thecooling chamber does not necessarily represent a bulk or equilibriumtemperature of the material in the cooling chamber. An appropriate rateof cooling may be provided in the cooling chamber by use of heatexchange fluids such as water or water glycol mixture, and the bulkmaterial held in the cooling chamber for a time and until a temperatureis reached at which it is observed that removal of the bulk materialfrom the chamber does not result in significant increase in diameter.Thus, in various embodiments, cooling to a solidification temperatureof, for example, 90° F. or 30° C. means leaving the extruded bulkmaterial in the cooling chamber until the thermocouple embedded in thewalls of the cooling chamber reads 90° F. or 30° C. As noted, it hasbeen found that such a cooling period suffices for removal of the bulkmaterial, even though the bulk equilibrium temperature of the interiorof the bulk material could be higher than the measured temperature.

In various embodiments, the extruded bulk material is held in thecooling chamber for an additional period of time, such as 10 minutes,after the embedded thermocouple reads 90° F. or 30° C. The additionalcooling period can enable the cooled material to be more easily removedfrom the cooling chamber. In one embodiment, when the thermocouplereaches a reading of 30° C., a programmable logic controller (plc)starts a timer that in turns gives a signal when the desired time haspassed. At that time an operator can remove the compression deformedcrosslinked material from the chamber, or rams or other suitable devicescan be actuated to effect removal.

Sacrificial Puck

In a preferred embodiment, a so-called sacrificial puck is used toimprove the efficiency of the extrusion process. In referring to FIG. 3,a ram 30 is provided in a retracted position with respect to thedeformation chamber 2. FIG. 3 b shows the ram 30 retracted and thedeformation chamber 2 filled with a rod-like bulk material 50 and asacrificial puck 40. The sacrificial puck 40 is made of a crosslinkedpolymer, which may be the same as the crosslinked polymer of the bulkmaterial 50. It is preferably of approximately the same cross-sectionalshape and area as the bulk material 50 to be extruded. In FIG. 3 c, theram 30 is shown pushing on the sacrificial puck 40, which in turn pusheson the bulk material 50 to move the bulk material 50 through thereducing die 6 into the cooling chamber 4. FIG. 3 d shows the situationat the end of the stroke of the ram 30. The bulk material 50 is sittingcompletely in the cooling chamber 4, while the sacrificial puck 30occupies the reducing die 6. Upon retraction of the ram 30 as shown inFIG. 3 e, the sacrificial puck 40 tends to return to its originaldimension because it is not being cooled in the cooling chamber as thebulk material 50 is. As a result, the sacrificial puck tends toextricate itself from the reducing die as shown in FIG. 3 f. Thesacrificial puck 40 can then be removed from the deformation chamber andthe process repeated after a cycle time in which the bulk material 50cools to a suitable solidification temperature as discussed above.

Stress Relieving

Following extrusion and optional cooling to a solidificationtemperature, the bulk material is then preferably stress relieved. Inone embodiment, stress relieving is carried out by heating to a stressrelief temperature, preferably below the melting point of the polymericbulk material. If the cooling in the previous step is carried out whilemaintaining deformation force, the bulk material on stress relievingtends to expand and return to a dimension close to its originaldimension. In the non-limiting example of an extruded rod, as the bulkmaterial is heated, the diameter d₃ of the rod tends to increase to adiameter approaching d₁ of the original bulk material. In variousnon-limiting embodiments, it has been observed that the bulk materialretains about 90-95% of its original dimension upon stress relieving orstress relief heating.

The stress relief process tends to run faster and more efficiently athigher temperatures. Accordingly, stress relief temperatures close tobut less than the melting temperature are preferred, for example fromthe melting point to the melting point minus 30 or 40° C. For UHMWPE,preferred stress relief temperatures include in the range of about 100°C. to about 135° C., 110° C. to about 135° C., 120° C. to 135° C., andpreferably 125° C. to about 135° C.

Stress relieving is carried out for a time to complete the stress reliefprocess. In various embodiments, suitable times range from a few minutesto a few hours. Non-limiting examples include 1 to 12 hours, 2 to 10hours, and 2 to 6 hours in an oven or other suitable means formaintaining a stress relief temperature. Although the stress relievingcan be carried out in a vacuum, in an inert atmosphere, or in a packagedesigned to exclude an atmosphere, it is preferably carried out in anair atmosphere.

Under some conditions, the solidified extruded bulk form exhibits atendency to bend or other deviate from a preferred straight or linearorientation during the heating or other treatment associated with stressrelieving. To counter this tendency, in one embodiment, the bulkmaterial is held in a mechanical device that functions to keep the bulkmaterial straight (measured on the axial direction) during the stressrelieving step. In a non-limiting example, the bulk material is placedinto V-channels to keep them straight. For example, several V-channelsare equally spaced from each other and are part of the same physicalstructure. The several V-channels may, for example, be welded to thestructure at equal spacings. The extruded bars are positioned on abottom set of V-channels and then another set of V-channels is set ontop of the extruded bars to rest on top of the bars. These channels helpto keep the bars straight during stress relieving.

In various embodiments, the product of the crosslinking, heating,compressing, cooling and stress relieving steps is a bulk materialhaving dimensions approximately equal to the original bulk materialbefore crosslinking. As a result of the steps taken on the bulkmaterial, the bulk material exhibits high tensile strength in the axialdirection, a low but detectable level of free radical concentration, anda high degree of resistance to oxidation.

The process described can be followed with regard to the dimensions ofthe crosslinked polymer at various stages of the process. In variousembodiments, a bulk material having an original dimension or diameter ofd₁ is crosslinked and heated to a compression deformation temperature.The crosslinked heated material is then compressed to a dimension ordiameter d₂ which is less than d₁. In an optional step, the material isthen held while cooling at a diameter d₃ that may be the same as d₂, butin any case is less than the original dimension or diameter d₁. Aftercooling, stress relieving returns the bulk material to a diameter d₄which is greater than d₃ and in some embodiments is approximately equalto the original dimension or diameter d₁. For example, if the originalbulk material is a 3″×14″ cylinder of UHMWPE, the treated preformresulting from the steps above preferably typically has a diameter ofabout 2.7 to 3 inches.

Following the treatment steps described above, the bulk materialcharacterized by an axial direction is machined according to knownmethods to provide bearing components for implants. In the case of acylindrical treated bulk material perform, it is preferred first to turnthe outer diameter of the cylinder to remove any oxidized outer layersand to provide a straight and round cylinder for further processing. Ina preferred embodiment, the cylinder is then cut into billets along theaxial direction, and each billet is machined into a suitable bearingcomponent. Preferably, the bearing components are machined from thebillets in such a way that the in vivo load bearing axis of the bearingcomponent corresponds to the axial direction of the bulk preform fromwhich it is machined. Machining this way takes advantage of theincreased tensile strength and other physical properties in the axialdirection of the preform.

For example, in bearing components for joint replacements, the stressesat the bearing surface are typically multiaxial, and the magnitude ofthe stresses further depends on the conformity of the joint. For hipapplications, the polar axis of the cup is aligned with the longitudinalaxis of the extruded rod, corresponding to the axial direction. The wallof the cup, at the equator and rim, is parallel to the long axis of therod, and will benefit from the enhanced strength in this directionduring eccentric and rim loading scenarios.

Oxidative Resistance

It has been found that UHMWPE, preforms, and bearing components madeaccording to the invention have a high level of oxidative resistance,even though free radicals can be detected in the bulk material. Tomeasure and quantify oxidative resistance of polymeric materials, it iscommon in the art to determine an oxidation index by infrared methodssuch as those based on ASTM F 2102-01. In the ASTM method, an oxidationpeak area is integrated below the carbonyl peak between 1650 cm⁻¹ and1850 cm⁻¹. The oxidation peak area is then normalized using theintegrated area below the methane stretch between 1330 cm⁻¹ and 1396cm⁻¹. Oxidation index is calculated by dividing the oxidation peak areaby the normalization peak area. The normalization peak area accounts forvariations due to the thickness of the sample and the like. Oxidativestability can then be expressed by a change in oxidation index uponaccelerated aging. Alternatively, stability can be expressed as thevalue of oxidation attained after a certain exposure, since theoxidation index at the beginning of exposure is close to zero. Invarious embodiments, the oxidation index of crosslinked polymers of theinvention changes by less than 0.5 after exposure at 70° C. to fiveatmospheres oxygen for four days. In preferred embodiments, theoxidation index shows a change of 0.2 or less, or shows essentially nochange upon exposure to five atmospheres oxygen for four days. In anon-limiting example, the oxidation index reaches a value no higher than1.0, preferably no higher than about 0.5, after two weeks of exposure to5 atm oxygen at 70° C. In a preferred embodiment, the oxidation indexattains a value no higher than 0.2 after two or after four weeksexposure at 70° to 5 atm oxygen, and preferably no higher than 0.1. In aparticularly preferred embodiment, the specimen shows essentially nooxidation in the infrared spectrum (i.e. no development of carbonylbands) during a two week or four week exposure. In interpreting theoxidative stability of UHMWPE prepared by these methods, it is to bekept in mind that the background noise or starting value in theoxidation index determination is sometimes on the order of 0.1 or 0.2,which may reflect background noise or a slight amount of oxidation inthe starting material.

Oxidation stability such as discussed above is achieved in variousembodiments despite the presence of a detectable level of free radicalsin the crosslinked polymeric material. In various embodiments, the freeradical concentration is above the ESR detection limit of about0.06×10¹⁵ spins/g and is less than that in a gamma sterilized UHMWPEthat is not subject to any subsequent heat treatment (aftersterilization) to reduce the free radical concentration. In variousembodiments, the free radical concentration is less that 3×10¹⁵,preferably less 1.5×10¹⁵, and more preferably less than 1.0×10¹⁵spins/g. In various embodiments, the oxidation stability is comparableto that of melt processed UHMWPE, even if according to the invention theUHMWPE is processed only below the melting point.

Although the invention is not to be limited by theory, the free radicalsin the deformation processed UHMWPE described above may be highlystabilized and inherently resistant to oxidative degradation.Alternatively or in addition, they may be trapped within crystallineregions of the bulk material and as a consequence may be unavailable toparticipate in the oxidation process. Because of the oxidation stabilityof the material, in various embodiments it is justifiable to employ gaspermeable packaging and gas plasma sterilization for the extrusionprocessed radiation UHMWPE. This has the advantage of avoiding gammasterilization, which would tend to increase the free radicalconcentration and lead to lower oxidation stability.

In various embodiments, the solid state deformation process providespolymers that are characterized by a crystal and molecular orientation.By molecular orientation is meant that polymer chains are orientedperpendicular to the direction of compression. By crystallineorientation it is meant that crystal planes in polyethylene, such as the200 plane and the 110 plane are oriented to the direction parallel tothe compression plane. In this way the crystal planes are oriented. Thepresence of the orientations can be shown by means of birefringentmeasurements, infrared spectra, and x-ray diffraction.

The plane of compression for articles compressed in a radial directionis understood to be a surface surrounding and parallel to the radialsurface of the bulk material that is processed according to theinvention. In the non-limiting example of a cylindrical rod, a sequenceof circular cross sections along the axial direction defines a radialsurface and a compression plane perpendicular to that surface. Inresponse to compression around the radial plane, polymer chains orientthemselves perpendicular to the direction of compression. This has theeffect in a cylinder of providing molecular orientation generallyparallel to the radial plane. It is believed that with this molecularand crystal orientation contributes to the enhancement of mechanicalproperties, and to anisotropy in the mechanical properties with respectto the axial and transverse (or radial) directions.

In various embodiments, crosslinked UHMWPE are provided that exhibit ahigh level of tensile strength in at least one direction.Advantageously, bearing components and implants are provided that takeadvantage of the increased strength of the bearing material. Forexample, in crosslinked UHMWPE, it is possible to achieve a tensilestrength at break of at least 50 MPa, preferably at least 55 MPa, andmore preferably at least 60 MPa. In various embodiments, materials areprovided with a tensile strength at break in the range of 50-100 MPa,55-100 MPa, 60-100 MPa, 50-90 MPa, 50-80 MPa, 50-70 MPa, 55-90 MPa,55-80 MPa, 55-70 MPa, 60-90 MPa, 60-80 MPa, and 60-70 MPa. In anon-limiting embodiment the tensile strength of a UHMWPE preparedaccording the invention is about 64 MPa in the axial direction.

EXAMPLES Comparative Example

Isostatically molded UHMWPE bar stock (Ticona, Inc., Bishop, Tex.) ispackaged in an argon environment and gamma sterilized to a dose of 25 to40 kGy

Example 1

Radiation crosslinked, deformation processed UHMWPE is produced usingthe following steps:

1. Radiation crosslinking. Isostatically molded UHMWPE rods ofdimensions 3″×14″ (Ticona, Inc., Bishop, Tex.) are vacuum packed in afoilized bag and gamma radiation crosslinked with a nominal dose of 50kGy.

2. Preheating Prior to deformation processing, the rod is removed fromthe foilized bag and raised to 133° C. for 4 to 12 hours in an oven.

3. Solid state, hydrostatic extrusion. The heated rod is then removedfrom the oven and placed in the holding chamber of a press. Thetemperature of the holding chamber is 130° C.±5° C. The bar is then ramextruded using a sacrificial puck made of crosslinked UHMWPE through acircular die, into a cooling chamber with a diametral compression ratioof 1.5 (diameter of 3″ down to 2″).

4. Cooling and solidification. The cooling chamber is sized so as tomaintain the extruded rod in a deformed state. The walls of the coolingchamber are water-cooled. When thermocouples embedded in the wall (about1 mm from the inside wall) read 30° C., the solidified rod is removed,optionally after an additional cooling period of ten minutes, in anon-limiting example. If desired, a second bar is ram extruded to ejectthe cooled bar from the cooling chamber, once the temperature reachesabout 30° C.

5. Stress relief, annealing. The deformed rod is then heated at 133±2°C. for 5 hours. The annealing also improves dimensional stability in thematerial. The rod is then slowly cooled to room temperature. Theextruded rod retains about 90-95% of its initial diameter after thestress relief step.

6. Gas plasma sterilization. After cooling, a liner or other bearingmaterial is machined and the machined part is non-irradiativelysterilized (e.g., with ethylene oxide or gas plasma)

Specimen Preparation and Orientation

For compression tests and accelerated aging, right rectangular prismspecimens are evaluated. The specimens measure 12.7 mm by 12.7 mm by25.4 mm (0.50 in. by 0.50 in. by 1.00 in.) They are machined from therod stock parallel (the axial direction) or perpendicular (thetransverse direction) to the long axis.

For tensile tests, dumbbell-shaped tensile specimens consistent with theType IV and V specimen description provided in ASTM D638-02a are tested.Specimens are 3.2±0.1 mm thick. Specimens are oriented parallel orperpendicular to the long axis, reflecting the axial and transversedirections, respectively).

Physical and Mechanical Properties

Tensile strength at break is determined according to ASTM 638-02a.

The concentration of free radicals in the UHMWPE materials ischaracterized using an ESR spectrometer (Bruker EMX), as describedpreviously in Jahan et al., J. Biomedical Materials Research, 1991; Vol.25, pp 1005-1017. The spectrometer operates at 9.8 GHz (X Band)microwave frequency and 100 kHz modulation/detection frequency, and isfitted with a high sensitivity resonator cavity. For a good spectralresolution and/or signal-to-noise ratio, modulation amplitude is variedbetween 0.5 and 5.0 Gauss, and microwave power between 0.5 and 2.0 mW.

Accelerated Aging

Specimens are aged in 5 atmospheres of oxygen in accordance with ASTM F2003-00. Some specimens are aged for two weeks according to thisstandard, and others are aged for four weeks. Aging is performed in astainless steel pressure vessel. The specimens are chosen and orientedsuch that the tested axis is vertical. Thus, the top and bottom facesare perpendicular to the test axis. The top face is labeled for lateridentification. The vessels are then filled with oxygen and purged fivetimes to ensure the purity of the aging environment. The prisms rest ona flat surface inside the pressure vessel; thus each prism's bottom faceis not exposed to oxygen, but each of its other faces are exposed tooxygen throughout the aging period.

The vessel is placed in the oven at room temperature (24±2° C.), and theoven was heated to the aging temperature of 70.0±0.1° C. at a rate of0.1° C./min.

FTIR Analysis

Materials are evaluated before and after accelerated aging by Fouriertransform infrared spectroscopy (FTIR) in transmission (Excalibur seriesFTS3000 with a UMA-500 microscope attachment; Bio-Rad Laboratories,Hercules, Calif.). FTIR profiling is conducted perpendicular to thetransverse direction.

Oxidation index measurement and calculations are based on ASTM F2102-01. Oxidation peak area is the integrated area below the carbonylpeak between 1650 and 1850 cm⁻¹. The normalization peak area is theintegrated area below the methylene stretch between 1330 and 1396 cm⁻¹.Oxidation index is calculated by dividing the oxidation peak area by thenormalization peak area.

Results

Data for the Comparative Example and Example 1 are given in the Table

Example Comparative Example 1, 1, Example Example 1 axial transverseTensile Strength 46.8 ± 2.0  64.7 ± 4.5 46.1 ± 3.5 at Break [MPa] Freeradical 3.82 × 10¹⁵  0.22 × 10¹⁵ concentration, spins/g Oxidation index0.2 <0.1 before aging (at surface) Oxidation index 1.2 <0.1 after aging(at surface)

Although the invention has been described above with respect to variousembodiments, including those believed the most advantageous for carryingout the invention, it is to be understood that the invention is notlimited to the disclosed embodiments. Variations and modifications thatwill occur to one of skill in the art upon reading the specification arealso within the scope of the invention, which is defined in the appendedclaims.

1. A medical implant comprising a bearing component made by machining agamma-crosslinked UHMWPE, wherein the gamma-crosslinked UHMWPE is in theform of a cylinder characterized by an axial direction and a transversedirection orthogonal to the axial direction, wherein the axial directioncorresponds to the longitudinal direction of the cylinder, wherein thetensile strength in the axial direction is at least 60 MPa and whereinthe tensile strength in the axial direction is 20% or more greater thanthe tensile strength in the transverse direction, and wherein thebearing component has a free radical concentration greater than0.06×10¹⁵ spins/gram.
 2. A medical implant according to claim 1, whereinthe tensile strength in the axial direction is 60-100 MPa.
 3. A medicalimplant according to claim 1, wherein the tensile strength in the axialdirection is 60-90 MPa.
 4. A medical implant according to claim 1,wherein the tensile strength in the axial direction is 60-80 MPa.
 5. Amedical implant according to claim 1, wherein the oxidation index of thebearing component shows no measurable increase on exposure to 5 atmoxygen at 70° C. for 2 days.
 6. A medical implant according to claim 1,wherein the oxidation index of the bearing component shows no measurableincrease on exposure to 5 atm oxygen at 70° C. for 4 days.
 7. A medicalimplant according to claim 1, wherein the bearing component is anacetabular cup.
 8. A medical implant according to claim 1, wherein thebearing component is a tibial plateau.
 9. A medical implant according toclaim 1, wherein the bearing component comprises UHMWPE having aconcentration of free radicals greater than 0.06×10¹⁵ spins/g and lessthan 3×10¹⁵ spins/g, and stable to oxidation for 4 weeks in 5 atm oxygenat 70° C., as measured by an increase in the oxidation index of 0.1 orless.
 10. A medical implant according to claim 1, wherein the bulkmaterial is in the form of a cylindrical rod with a diameter of fromabout 2 inches to about 4 inches.
 11. A medical implant according toclaim 1, wherein the implant is an artificial hip joint.
 12. A medicalimplant according to claim 1, wherein the UHMWPE is crosslinked with agamma-irradiation dose of from 0.1 to 10 Mrad.
 13. A medical implantaccording to claim 1, wherein the UHMWPE is crosslinked with agamma-irradiation dose of from 0.1 to 5 Mrad.
 14. A bearing componentfor an artificial joint machined from a crosslinked UHMWPE, wherein thecrosslinked UHMWPE is in the form of a cylinder characterized by anaxial direction and a transverse direction orthogonal to the axialdirection, wherein the axial direction corresponds to the longitudinalaxis of the cylinder wherein the tensile strength in the axial directionis at least 60 MPa and wherein the tensile strength in the axialdirection is 20% or more greater than the tensile strength in thetransverse direction, and wherein the bearing component in use has aload bearing axis substantially coincident with the axial direction ofthe cylinder, and wherein the bearing component has a free radicalconcentration greater than 0.06×10¹⁵ spins/gram.
 15. A bearing componentaccording to claim 14, wherein the tensile strength in the axialdirection is 60 to 100 MPa.
 16. A bearing component according to claim14, wherein the tensile strength in the axial direction is 60-90 MPa.17. A bearing component according to claim 14, wherein the tensilestrength in the axial direction is 60-80 MPa.
 18. A bearing componentaccording to claim 14, wherein the tensile strength in the axialdirection is 60-70 MPa.
 19. A bearing component according to claim 14,wherein the bearing component has an oxidation index that shows nomeasurable increase on exposure to 5 atm oxygen at 70° C. for 2 days.20. A bearing component according to claim 14, wherein the bearingcomponent has an oxidation index that shows no measurable increase onexposure to 5 atm oxygen at 70° C. for 4 days.
 21. A bearing componentaccording to claim 14, wherein the bearing component is an acetabularcup.
 22. A bearing component according to claim 14, wherein the bearingcomponent is a tibial plateau.
 23. A medical implant according to claim1, wherein the gamma-crosslinked UHMWPE is in the form of a cylinderhaving a circular cross section perpendicular to the axial direction.24. A bearing component according to claim 14, wherein the crosslinkedUHMWPE is in the form of a cylinder having a circular cross sectionperpendicular to the axial direction.